This invention relates, in general, to semiconductor processing and, more particularly, to a process for flowing and or re-flowing doped insulator layers utilized in the manufacture of semiconductor devices.
In the manufacture of semiconductor devices it is usually necessary, at least once during the process, to heat an insulating layer in order to cause the layer to soften and "flow". One example of such a flow step is detailed in U.S. Pat. No. 3,833,919 which issued on Sept. 3, 1974 and in U.S. Pat. No. 3,925,572 which issued on Dec. 9, 1975, both to C. T. Naber. In both references an undoped silicon dioxide (SiO.sub.2) insulator layer is deposited on a lower level of conductors and a layer of heavily doped SiO.sub.2 is formed over the undoped layer. The doped layer, when heated, will soften and flow at about 1000.degree. C. the flow temperature being heavily dependent on the concentration of the dopant in the doped layer. However, the temperature is sufficiently lower than 1300.degree. C. required to cause the undoped layer to soften and flow.
Similarly, it is frequently necessary to reheat a layer that was made to flow and thus cause it to "re-flow." This re-flow process is often necessary when, after etching the flow layer to form contact openings, the re-flow process step rounds the sharp edges formed in the glass. Thus, some important reasons for utilizing low flow and re-flow temperatures is (1) to minimize the possibility of diffusing the dopant out of the heavily doped SiO.sub.2 layer and into the substrate and (2) to minimize source-drain side diffusion to thus produce longer electrical channel lengths. Conversely, over-shortening of the channel length is minimized.
Another beneficial effect of using lower flow/re-flow temperatures is the resultant enhanced radiation tolerance that the device exhibits. This latter benefit is described in detail in U.S. Pat. No. 4,259,779 which issued on Apr. 7, 1981 to A. C. Ipri, et al. This patent describes a method of making a radiation hardened (RAD hard) transistor by forming the channel oxide at a temperature under 1000.degree. C. and thereafter performing the remaining process steps at or below 900.degree. C. Thus, the need for a low temperature semiconductor manufacturing process is of importance to the semiconductor manufacturing industry.